The present invention relates generally to mounting surface mount devices (SMD). More specifically, the invention relates to curing a mounting adhesive and reflow soldering of SMD's using minimal processing time and equipment.
Surface mount device technology usage has become more prevalent in the electronics industry, especially in view of the ever-increasing need for smaller modules. SMDs have two or more solder pads on either end of the device. To mount the SMD to a circuit board, solder paste is dispensed on solder pads of the circuit board. Dispensing typically is accomplished by screen printing or stencil printing. Then the solder pads of the SMD are stacked on the screen-printed solder paste. The solder paste is then heated to reflow the solder to a molten state so that the solder pads on the SMD and on the circuit board are joined when the solder resolidifies upon cooling. However, during the reflow of the solder, significant forces act upon the SMD causing the SMD to move. This is especially true for small SMDs.
To remedy component movement, various adhesives are used to secure the SMD to the circuit board prior to and during the reflow process. One disadvantage in using adhesives is that they typically require substantial time to cure so that the SMD is securely held in place. One type of adhesive is thermally-cured adhesive, which typically requires up to 15 minutes in a curing oven to fully cure. Other types of adhesive may be cured using ultraviolet radiation, which also requires a significant amount of time to cure, as well as requiring specialized equipment.
Once the adhesive is cured the circuit board is cooled. The circuit board with the attached SMD is then placed in a reflow oven, and the solder is heated to its molten state to wet the contacts of the solder pads of the SMD and the solder pads of the circuit board. While the SMD is in the reflow oven, the circuit board assembly is first heated to a temperature to activate the flux present within the solder paste. Then, the assembly is heated to a second, higher temperature after the flux has been activated to reflow the solder for a predetermined amount of time.
One drawback to the use of adhesives has been that a station separate from that required to provide heating reflow is required to either provide thermal curing or ultraviolet curing. Since it is also desirable to process the circuit board with the SMD in a continuous manufacturing assembly process, eliminating extra time consuming steps and equipment is highly advantageous.
Another drawback to the use of the adhesive curing process is that the solder joints are less reliable. Since solder is typically dispensed before curing, during curing the solvents and activators within the solder paste are either changed or evaporated. Consequently, when the solder is reflowed the properties of the solder paste are typically affected.
A disadvantage of processing small SMDs is that the board geometry is highly susceptible to arc-over and electromigration. These situations occur when electrostatic charge differential is built up between two adjacent solder pads and is released. Arc-over occurs from a sudden burst of electrical energy. Electromigration is when the charge is slowly conducted over the surface of the circuit board between the two solder pads as a result of ionic contamination and biased potential.